In two new studies published this week, neuroscientists have manipulated brain cells to create false memories in mice.

Understanding how memories are created offers some insight into Alzheimer’s, schizophrenia, and post traumatic stress disorder, the researchers say.

Memories are thought to be formed and stored in specific populations of neurons in the brain. Stimulating some regions can trigger behaviors and recollections. In the early 1900s, neurosurgeon Wilder Penfield treated epilepsy patients by scooping out parts of the brain where seizures originated. He stimulated parts of the brain with tiny jolts of electricity and patients reported recalling complex events.

In these new 21st-century redesigns, the researchers created false memories by making mice recall fearful situations of mild electrical shock. By activating artificial memory in the mice, they reconditioned them to fear electric shocks even when there were none.

They genetically engineered mice with neurons that expressed light-activated proteins (a technique called optogenetics). Specifically, they coupled the light-activated protein with the set of neurons that fired only when a mouse was learning about a new environment and forming memories.

The mice were put into a box and allowed to explore. As they formed memories of that box, the neurons involved became responsive to light.

After exploring the box, they received a mild electric shock to the foot – so that they learned to fear the environment where the shock occurred.

Then they were put into a different box, and pulses of light were delivered to their brains through implanted optic fibers.

When exposed to the light in a different environment, the neurons involved in the fear memory switched on – and the mice quickly froze into a defensive crouch, suggesting how they actually recalled the memory of being shocked. They perceived this replay of a fearful memory – even though the memory was artificially reactivated.

They genetically engineered mice so that neurons that fired while a memory was made would fire again when the brain was injected with a drug.

The mice were put into a box, and their neurons formed a memories as they explored. White walls and floor, no particular odors. Because these neurons fired during memory-making, they could be reactivated when the drug was injected, allowing the researchers to induce an involuntary memory of the box.

When the mice were put into a second box, they were given a drug injection – which made them remember the first, white box.

In that second box (wintergreen-scented with a black-and-white checkered wall and a gridded floor), they were given a small electric shock.

Reactivating the memory of the white room during the shock session taught the mice to associate the combination of the reactivated memory and the black-and-white room with a shock – forming a hybrid memory.

As a result, the mice developed a fear of a mixture of both boxes – an artificial memory of an imaginary environment. The only time they froze was when they were in the second, black-and-white box and the drug was injected.

Janet Fang has written for Nature, Discover and the Point Reyes Light. She is currently a lab technician at Lamont-Doherty Earth Observatory. She holds degrees from the University of California, Berkeley and Columbia University. She is based in New York.
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